The use of high rotation engines and the introduction of a pressurized lubrication system in internal combustion engines require the use of a specific ring, or oil flow control ring, to control the large amounts of oil thrown by the connecting rods against the cylinder walls.
The oil flow control ring presents, on the external face thereof, two surfaces in contact with the cylinder wall, which are separated by a channel containing radial slots for oil flow drainage.
The oil flow control ring must present two main functional characteristics: scraping the oil from the cylinder wall towards the engine crankcase and maintaining a sufficient amount of oil for the compression rings there above, in order to maintain the oil film between the rings and the cylinder all the time. In some known constructions, the oil-control ring is formed of an expanding element and a pair of parallel ring segments spaced from each other by the expanding element, which maintains said ring segments acting against the cylinder wall.
The total force of the ring is composed of the force of the ring segments, added by the force provoked by deflection of the expander when mounted to the cylinder. The expander operates as a spring. In rest conditions, the expander has a diameter larger than the diameter of the cylinder. When compressed to the diameter of the cylinder, it suffers a deflection, jointly with the ring segment.
Due to the requirements for the internal combustion engines to present smaller gaps and more controlled wear, it is necessary to avoid the occurrence of relative movement between the expander and the ring segments, which can increase the axial gap.
Engine tests made either in dynamometers or in the field have recently shown the increasing need for maintaining or controlling the increase of the gaps between the expander and the ring segments.
The relative movement between the expander and the ring segments of the multiple oil-control ring is characterized by the rotation of at least one ring segment around the expander caused by the increase of the axial gap between the ring segment and the piston groove, resulting from wear and the consequent fail of the expander to deflect axially, said expander being unable to make the ring segment overcome, by axial force, such gap increase, leaving it free to rotate around the expander by the action of gases, oil film and attrition. The radial sealing of the ring is thus impaired, reducing the contact force between the ring segment and the cylinder, increasing degradation of the engine and consequently increasing the engine indices, such as consumption of lubricant oil, whose excess is not duly scraped to the crankcase, being exposed to the action of hot gases and consequently burnt in the combustion chamber or dragged to the exhaust system, turning the system tribologically vulnerable.
Besides not providing the adequate ring-cylinder sealing, the relative movement between the component parts of the ring may increase the wear of the ring segment-expander couple, which usually occurs between the inner diameter of the segment and the spacer support. Such wear is also associated with less deflection of the expander, reducing the sealing force against the cylinder.
Upon assembling the ring segments and expander in the groove, the forces of the ring segments applied onto the stops of the expander provoke a deflection in the expander body, pushing the ring segments against the groove sides (known as lateral sealing). With wear, there is an increasing reduction in the bending force of the expander, in its force to push the ring segments against the groove walls, and in the radial force of the ring segments against the sidewalls of the cylinder, increasing the probability of rotation, allowing the ring segments to rotate in relation to the expander, further increasing the wear.
The existence of a relative rotary movement between the ring segments and the expander provokes wear in the mutually seating surfaces, that is, between the inner surface of each ring segment and the expander support, known as secondary wear, which produces a gap between the parts with relative movement, since it wears both materials of the mutually contacting parts, reducing the deflection and strength of the ring.
Such wear modifies, with time, the diametral difference between the expander and the cylinder wall, taking into account the ring, reducing the tangential force of the ring, thus reducing the scraping force, as there is less bending.
Besides the reduction of force, the relative movement causes wear in a superficial reinforcement coating, for material hardening, which both the expander and the ring segments receive by treating the respective surface by nitration. This wear is a factor that leads the engine manufacturers to refuse many pieces.
In some known constructions, the rotational locking between the ring segments and the expander occurs by providing an anti-rotating structure, in which an end portion of the expander usually presents a bending, producing an axially projecting tab that operates in a slot defined in the ring segment. In these solutions, the bending may be partial or total in the radial width of one or of both the end portions of the expander (JP332156, JP525064).
While the solution above obtains a rotational locking of the ring segments in relation to the expander, it has some disadvantages, such as a mechanical fragility in the region of the bendings, as well as an operational fragility of the ring, due to the deterioration of the nitration in said region, in the cases in which the oil-control ring is submitted to said treatment.
The oil-control ring constructions present two types of solution for the ring segments of said oil-control ring: said ring segments are either made of steel provided with a chrome coating, or they are nitrided, in which case the whole periphery of the ring segment receives a treatment before nitration, such as the oil-control rings disclosed in Japanese patents JP332156 and JP525064. In these constructions, the level of stress in the bending region increases the possibility of occurring cracks in this region, which increase with the nitration, if the latter is carried out after the provision of the bends.
On the other hand, the provision of bends, after submitting the ring to nitration, impairs the superficial hardening of the ring segment, jeopardizing the contact of the ring with the piston and cylinder.